Since the cost of a well increases with the bore diameter, it is desirable for pump assemblies to be of small external diameter and high rotational speed in order to secure the same pump performance as with a pump of lower rotational speed but of larger external diameter. For such rotational speed the pump assembly must be energised by way of an appropriate system for example by way of a frequency changer.
There is a present trend towards combining all the structural units of a pump assembly within a single structure.
According to EP-A-No.0100 390 three pump elements consisting of the motor, the frequency changer and a pumping stage are combined within a single casing, the pump being of the single stage kind, the rotor of which is selected as a function of the required speed of rotation of the pump. The frequency changer is disposed beside the motor laterally thereof. Such a pump assembly is, however, of relatively complex construction and is, therefore, costly to produce and is unsuitable for use as an underwater motor pump of high performance, for small diameter wells.